Abstract We investigated the degradation of neutral beam (NB) fast-ion confinement depending on the NB power without magnetohydrodynamics instabilities in the Large Helical Device (LHD). In the LHD deuterium experiment, the neutron emission rate per NB power decreased by up to 20% with increasing injected NBs during a single discharge. Because there were no significant variations in the electron temperature and density, the NB shine-through rate, or the magnetic fluctuation due to the change in NB power, the reduction in the neutron emission rate indicates the degradation of the fast-ion confinement. In this paper, we formulated this degradation depending on the NB power and quantitatively estimated the degraded effective confinement time. In addition, we performed neutron emission rate simulations using the obtained effective confinement time. The simulation and experimental results were in good agreement, suggesting that the degraded effective confinement time is valid.